In recent years concrete and mortar systems have been increasingly used in flowable, fluid, or self-consolidating consistencies in an attempt to increase the speed of construction. One method that is used to facilitate placement of these high slump mixtures is pumping. Pumped cementitious compositions such as concrete, mortar or grouts are conveyed by pressure through either rigid pipes or flexible hoses and discharged directly into the required area. Pumping may be used for most concrete construction, but it is especially useful where space or access for construction equipment is limited.
Under pressure and motion, a small amount of water is transmitted from the cementitious composition to the sidewall of the pipe in order to provide a lubricating layer. The only element of a cementitious composition that is pumpable is water, and it is the stability of the water in the mix under pressure that largely determines pumpability. When a cementitious composition blocks a pipeline there is a process of water loss, that is, a process in which the prevailing axial pressure in the pipe forces the water to move faster than the cementitious composition, leaving a portion of it drier than needed for the saturated state. Addition of cement and fine aggregates having grain sizes of less than 300 μm inhibits the migration of water under pumping pressure. Sometimes high contents of cement and fine aggregates increase the flow resistance, the reason being that such mixtures have too much cohesiveness, which produces a high viscosity that prevents the development of an adequate lubricating layer.
After placement of the cementitious composition, the mixture must also remain homogeneous. Settlement of the denser portions of the mixture can involve separation of water (bleeding) as well as stratification of the coarse fractions due to density differences (segregation). Bleeding causes a high water to binder ratio at the top surface of the cementitious composition, as well as deposition of an excess proportion of line material in the upper region of the cementitious composition. This causes lower strength, higher shrinkage, and in applications where the upper surface must support load (such as grouting), the inability of the material to function as intended. Likewise, segregation of the aggregate portion of the cementitious composition produces an inhomogeneous composition with differing properties throughout the installed cementitious composition.
The increase in slump of mortar and concrete mixes can also increase the tendency to lose water as well as allow the settlement of the coarse particles of the mixture (segregation). This water separation from the mixture is known as “bleeding”. Both bleeding and segregation can happen under the influence of gravity, as well as be increased through the use of additives commonly used to improve fluidity.
Admixtures may help the process of pumping, such as water-reducing admixtures like sulfonated naphthalene-formaldehyde condensate (SNF) which reduce the flow resistance when used with unchanged water contents, to obtain more fluid consistencies. Water-reducing admixtures improve the dispersion of the cement grains and minimize the agglomeration tendency.
A number of water-soluble polymers have long been used as thickening or stabilizing additives to minimize segregation and bleeding issues of cementing compositions. Such polymers could be alkyl derivatives of cellulose, for example methylcellulose, hydroxymethyl, hydroxyethyl, or hydroxypropyl cellulose, and the like, clays, guar gum, welan gum or other esterified polysaccharides. These water soluble polymers serve as a “suspending aid” by reducing the tendency of aggregates in fresh cementitious compositions to settle, and of mixing water to bleed on the surface. They provide an improved cohesion allowing aggregate particles to remain suspended in the cementitious composition without segregation when subjected to normal movement associated with transportation, pumping, placing, and consolidation by vibration. Nevertheless, all these traditional suspending aids have adverse effects such as impairing the initial workability by increasing water demand and/or viscosity, retarding the setting and hardening of fresh mixes, entraining air, and reducing flow retention.
Therefore, it is desirable to provide an admixture formulation that stabilizes the rheology by reducing bleeding and separation of a cementitious composition without adversely affecting workability, setting, strength and other performance properties.